The present invention relates to a power control method in a mobile communications system employing a hopping scheme on the radio connection. The invention further relates to a mobile communications system.
FIG. 1 in the accompanying drawing shows a simplified block diagram of the UMTS (Universal Mobile Telecommunication System). A mobile station (MS) communicates over the radio path with a base transceiver station (BTS), in the case of FIG. 1, with BTS1. The base station sub-system (BSS) consists of a base station controller (BSC) and base stations (BTS) under its control. A mobile services switching centre (MSC) usually controls a plurality of base station controllers BSC. The MSC communicates with other MSCs, and via a gateway mobile services switching centre, the UMTS network is connected to other networks such as the public switched telephone network PSTN, another mobile communications network PLMN or an ISDN network. The UMTS system is proposed to be implemented with the time division multiple access technique (TDMA) or with the code division multiple access technique (CDMA) or a combination of these two, i.e. a so-called hybrid system.
In digital radio systems implemented with the TDMA technique, such as the UMTS system, a group of mobile stations MS may, according to the time-division principle, use the same carrier frequency i.e. radio channel for communication with the base station BTS. The carrier is divided into successive frames that are further divided into timeslots, e.g. 8, 16 or 64 timeslots that are allocated to users as required. One frame lasts for 4.615 ms, which means that, in the case of eight timeslots, one timeslot lasts for 577 xcexcs. From the network point of view, one carrier may be used for establishing, for example, eight traffic channels.
Instead of duplex transmission implemented on two carrier frequencies, digital time-division radio systems may also carry time-division duplexing (TDD) transmission on one frequency. In such a case, at least one of the timeslots in the frame is assigned solely for uplink transmission and at least one for downlink transmission. The other timeslots in the frame are used, as need be, for either the uplink or the downlink communication.
Code division multiple access CDMA radio systems are based on spread spectrum communication. The data signal to be transmitted is multiplied by a special hash code assigned to the subscriber, whereby the transmission spreads out onto the broadband radio channel. This means that the same broadband radio channel may be used by several users for simultaneous transmission of CDMA signals processed with different hash codes. At the receiving end, the CDMA signal is despread by the subscriber""s hash code, whereby a narrow-band data signal is obtained. At the receiver, the other subscribers"" broadband signals represent noise by the desired signal. Therefore, the unique hash code of each subscriber in CDMA systems produces the traffic channel of the system in the same sense as the timeslot does in TDMA systems.
In mobile communications systems, the mobile station MS and/or the base station BTS carry out transmission power control to reduce the noise level in the network and to compensate for fading on the radio path. Power control usually aims at maintaining the received signal at almost the same, as low as possible a power level while maintaining the quality of the received signal. If the signal quality and/or level on the radio connection between the mobile communications network and the mobile station falls below the desired level, the transmission power may be adjusted at the base station BTS and/or the mobile station MS to improve the radio connection. The transmission power of the mobile station MS is usually adjusted from the fixed network by means of a special power control algorithm. The mobile station MS measures the received level (field strength) and the quality of the downlink signal received from the base station BTS1 of the serving cell, and the base station BTS1 of the serving cell, for its part, measures the received level (field strength) and quality of the uplink signal received from the mobile station MS. On the basis of these measurement results and power control parameters set, the power control algorithm determines a suitable transmission power level, which is then sent to the mobile station MS in a power adjustment command. Power control is continuously carried out during the call. In prior art TDMA systems, such as the GSM system, this typically takes place twice a second. An increase in the transmission power adds to the interference level in the network, which is why the aim is to keep transmission power levels as low as possible. Power control at the mobile station additionally contributes to reducing the power consumption of the mobile station.
Due to fading caused by reflections and multipath propagation of the signal transferred over the radio path, the amplitude of the received signal varies. In TDMA systems, particularly, fading makes signal transfer more difficult. To annul effects of fading, mobile communications systems employ not only power control but also, for example, frequency hopping and/or antenna hopping. The annulling effect of frequency hopping against fading is based on fading being frequency dependent. In antenna hopping, the transmission path of the signal changes, whereby the fading on the signal varies.
With the aid of frequency hopping, it is possible to reduce the co-channel interference caused by various base station signals, and effects on the signal to be transferred of fading on the radio path. In such a case, the frequency used on the radio connection is changed according to a predetermined frequency hopping pattern. Frequency hopping may be carried out as either baseband frequency hopping or transmitter-specifically as changes in the radio frequency. As shown by FIG. 2 of the accompanying drawings, hopping is usually carried out in periods of one burst (timeslot). FIG. 2 shows an example of frequency hopping on four radio frequencies on the radio connection between a mobile communications network and a mobile station. According to the hopping scheme of the figure, the successive bursts of the signal are transmitted on the frequency F4, F2, F3, F1, F2, F3, F4, F2, F3, etc.
It is additionally possible to reduce the effects of fading on the signal to be transmitted by means of antenna hopping, in which the signal is transmitted and/or received alternately via two or more antennas that are located physically apart. In such an event, the propagation path of the signal is different to each antenna. As fading is not only dependent on frequency but also on place, the changes in the propagation path may result in better propagation conditions. In antenna hopping, the transmitting and/or receiving antenna is changed according to a pre-set hopping pattern.
In TDMA systems, the received signal quality can also be improved with timeslot hopping, in which the signal is transferred on the radio connection in successive frames of the same carrier in a different timeslot according to a timeslot hopping pattern. Timeslot hopping effectively reduces short-time, periodic interference to the signal to be transferred, such as that caused by radio signals from other subscribers transmitting in the same timeslot. FIG. 3 shows an example of timeslot hopping when the signal is transmitted in successive frames in timeslot 1, 4, 0, 6, 1, 4, etc.
When transmitting speech or data in a digital communications system, transmission errors are developed on the transmission path that deteriorate the quality of the transmitted signal. Transmission errors are produced on the radio path when the signal becomes distorted e.g. due to multipath propagation, a interfering signal, or a high level of background noise. Error correction, such as channel coding or retransmission, and bit interleaving of the transmitted digital signal improve the transmission quality and tolerance for transmission errors. In channel coding, redundant information is added to the data to be transmitted, by means of which the original data may be detected without errors at the receiver even though the signal gained errors on the transmission path. Retransmission is employed for correction of transmission errors either independently or as an addition to e.g. channel coding, in which case the errors of a channel-coded transmission are corrected by re-transmitting the corrupted frames. In the interleaving of bits to be transmitted, the bits in several code words are mixed with one another whereby the adjacent bits of the signal are spread out to several bursts. Due to interleaving, the signal can in most cases still be detected even though an entire burst were lost during the transmission.
The problem with the prior art power control methods is that the power control is slow, particularly if the radio system employs a hopping method, for example frequency hopping. In frequency hopping, the fading of different frequencies may not correlate, resulting in that power control carried out according to one frequency is not good on the next transmission frequency. In the prior art TDMA systems, e.g. the GSM system, power control is based on average power calculated over several frequency hops and the adjusted transmission power remains the same during a number of bursts, i.e. on various hopping frequencies. A similar problem also occurs with the prior art power control method combined in conjunction with antenna hopping and timeslot hopping.
It is an object of the present invention to implement fast power control in a mobile communications system that employs a hopping technique.
This new type of power control is achieved with the method according to the invention, characterized by that which is disclosed in the independent claims 1, 8 and 15. The specific embodiments of the invention are disclosed in the dependent claims.
The invention further relates to a mobile communications system which, according to the invention, is characterized by that which is claimed in the independent claims 21, 22 and 23.
The invention is based on the idea that power control is advantageously carried out in synchronization with the hopping scheme employed on the radio connection and the transmission power is adjusted to suit each value set by means of the hopping, such as each hopping frequency, antenna, and timeslot. Power control is advantageously carried out based on the quality of the transmitted/received signal, set as a result of the previous, same hopping value, for example on the basis of the quality of the previous signal transmitted on the same hopping frequency, antenna and/or timeslot, or on the basis of the quality of the signal transmitted with the previous hopping value. In the first implementation of the invention, the aim is to amplify a faded or otherwise attenuated signal by increasing the transmission power, and in the second implementation of the invention, the aim is to amplify all other signals but the faded or otherwise attenuated signal by increasing their transmission power, with the power wasted in the faded or otherwise attenuated signal being minimized e.g. by reducing its transmission power.
Such a power control method provides the advantage that the transmission power required can effectively be minimized, whereby, when implemented in a mobile station, the power consumption of the mobile station is reduced.
The inventive method provides the further advantage of a lower overall level of interference in the network.